Higher operating temperatures for gas turbine engines are continually sought in order to increase efficiency. However, as operating temperatures increase, the high temperature durability of the components within the engine must correspondingly increase. Material processing for durability in high temperature applications yields a coarser grain microstructure with less fatigue resistance than fine grain structures at temperatures below about 649° C. There are numerous instances where operating conditions experienced by a component place differing materials property requirements on different portions of the component. A turbine disk for a gas turbine engine is an example of a type of component where tailored mechanical behavior in various portions of the article is preferred. Such disks are typically made from nickel-base superalloys, because of the temperatures and stresses involved in the gas turbine cycle. In the hub portion where the operating temperature is somewhat lower, the limiting material properties are often tensile strength and low-cycle fatigue resistance, which are superior in the fine grain condition up to about 649° C. In the rim portion where the operating temperature is higher because of the proximity to combustion gases, resistance to creep and hold time fatigue crack growth (HTFCG) is often the limiting material property. HTFCG is the propensity in a material for a crack to grow under cyclic loading conditions where the peak tensile strain is maintained at a constant value for an extended period of time. Therefore, processing the entire article to the damage-tolerant, coarse grain structure necessary for the high temperature rim location can result in a sacrifice of fatigue life at conditions encountered in the relatively cool-running bore.
The fatigue resistance of rotor alloys is an important measure in the design of turbine engine hardware. Conflicting requirements of other sizing criteria, such as rotor burst resistance, weight and available space, pose restrictions on the ability to satisfy fatigue life criteria. Life limiting locations are often localized to a zone or feature within a relatively long life region. An example is the inside diameter of a high pressure turbine disk bore at the mid-axial position due to constraint imposed by the mass of bore metal.
Accordingly, a process to locally improve fatigue performance of components without significantly affecting the overall balance of properties is needed.